The Château de Ramezay was built in 1705 as the residence of the governor of Montréal and by 1898 had been converted into a museum. The Historical Fancy Dress Ball was intended both as an entertainment and as an educational showcase for Canadian history and the history of the Château itself. Guests were required to portray a character from Canadian history and were divided into themed historical sets to perform specific dances, probably fancy quadrilles.
Harriet Wheeler chose to portray the Marquise de Vaudreuil, presumably Charlotte Fleury de la Gorgendière, wife of Pierre de Rigaud de Vaudreuil de Cavagnial, Marquis de Vaudreuil, the last Governor-General of New France (Canada) before it was lost to Britain in 1760. While her character would probably be fairly obscure nowadays to anyone but a Canadian, the outfit is a good example of a typical poudré costume. The museum's description of it states:
Mrs. Harriet Wheeler wore a pink satin hoop skirt trimmed with lace flounces and an overdress of pink and white flowered silk. Her patches -- small beauty spots on the face -- and powdered hair were frequently a part of 18th-century costumes.
Although guests were supposed to research their costumes and present accurate portrayals, there were varying degrees of success. To my eye, her corset looks rather more nineteenth century than eighteenth century, and the museum notes that:
The skirt of Mrs. Wheeler's supposedly 18th-century costume was held out with a circular hoop typical of the 1860s, and not the panniers that would have been more in keeping with the period. Nonetheless, the press praised her for wearing a "perfectly correct Pompadour costume." (Montreal Star, 19 January 1898.)